1. Field of the Invention
This invention relates generally to communication systems. More specifically, this invention relates to receivers used in spread spectrum, suppressed-carrier, bi-phase, phase shift keyed (PSK) communications systems.
While the present invention will be described herein with reference to a particular embodiment in a particular application, it is to be understood that those of ordinary skill in the art with access to the teachings of this invention will realize additional applications and embodiments within the scope thereof.
2. Description of the Prior Art
Spread spectrum techniques have proved to be invaluable in space, military and commercial communication systems where interference rejection, eavesdrop protection, signal hiding or access control is desired. As discussed by R. C. Dixon in Spread Spectrum Systems (1st Ed. 1976), a spread spectrum system is one in which the transmitted signal is spread over a frequency band much wider than that required to transmit the information being sent. The spreading is accomplished by modulation of the baseband signal with a carrier signal which is modulated, shifted or swept over a band of frequencies.
Carrier modulation or direct sequence is, for good reason, the most widely used spread spectrum technique. Moreover, bi-phase balanced modulation is the preferred carrier modulation technique because: (1) the carrier is suppressed and difficult to detect; (2) more power is available for sending information; (3) transmitted power efficiency is maximized; and (4) the modulator typically has a simple design with a low parts count.
The advantages of spread spectrum tranmission can not be realized without spread spectrum reception. Spread spectrum signals are usually demodulated in two steps. First, the spectrum-spreading modulation is removed and, second, the information bearing signal is demodulated. In direct sequence systems, the first step is accomplished by multiplication of the received signal by a local reference signal which is identical to the carrier in structure and synchronized in time. While structure identity is usually provided by design, synchronization is usually provided by a phase-locked loop. See Phase-Lock Techniques, by Floyd M. Gardner, pp. 217-230, (2nd Ed. 1979).
In a spread spectrum system the phase-locked loop must quickly acquire and accurately track the transmitted signal. To this end, the performance of such systems has been substantially enhanced by supplementing the phase-locked loop with a frequency acquisition circuit. Typical supplementary frequency acquisition circuits include sweep circuits and frequency discriminators. Donald Richmond develop such a circuit called a `quadricorrelator`. See "Color-Carrier Reference Phase Synchronization in NTSC Color Television", Proc. IRE, Vol. 22, pp. 106-133, January 1954.
Richmond's quadricorrelator differentiates and inphase signal component and multiplies the differentiated signal by a quadrature signal component to obtain a DC output proportional to the frequency difference between a locally generated reference oscillator and an applied synch signal.
Unfortunately, the output signal also includes a sinusoidal signal of equal amplitude at double the difference frequency. This double frequency component is undesirable and is difficult to filter out as the frequency difference term goes to zero in a closed loop system. Use of a low pass filter is undesirable insofar as it imposes a substantial penalty on the acquisition time of the quadricorrelator.
In addition, bi-phase PSK systems with suppressed-carrier may have a doppler induced frequency offset that must be removed for accurate reception. Thus, the present state of the art in spread spectrum systems is believed to be such that there is a need for a system that rapidly acquires and accurately tracks the center frequency of a spread spectrum suppressed-carrier bi-phase PSK signal.